1. Field of the Invention
The present invention relates to a liquid ejection head used for a thermal inkjet-printer head for ejecting liquid using thermal energy, a liquid ejection apparatus having the liquid ejection head, and a manufacturing method of the liquid ejection head. In detail, the invention relates to a technique in that the strain of liquid-ejection head components due to temperature variation is minimized so as to suppress characteristic degradation produced in the liquid ejection head.
2. Description of the Related Art
Among liquid ejection heads, in an inkjet-printer head employing a thermal system for an inkjet printer, a head chip is used having several hundreds of heater elements formed on a semiconductor substrate. While one head chip is used in the case of monochrome, in a color head, a two-block structure may be often adopted that is composed of a three-color head of Y (yellow), M (magenta), and C (cyan) integrally constructed at equal intervals and a K (black) head separately provided.
For increasing the printing speed, a number of liquid ejection parts (including nozzles, heater elements, and liquid chambers) may be provided within one head as many as possible, as one method. The liquid ejection part must have nozzles, heater elements, and liquid chambers as well as flow paths for communicating the entire liquid chambers together, so that the minimal area therefor is required.
Thus, at present, about 600 DPI (pitch of 42.3 μm) is assumed to be a limit. For example, a head having 256 liquid ejection parts at 600 DPI has a length of 10.8 mm. With increasing liquid ejection part size, the handling becomes difficult, reducing yield and increasing cost.
Accordingly, a thermal line head technique has been known in that a plurality of head chips are arranged so as to form one large line head as disclosed in Japanese Unexamined Patent Application Publication No. 2002-127427. By the structure mentioned above, a chip head having 320 heater elements at 600 DPI (15.4 mm length) is made, for example, so as to form a line head by arranging the 64 chip heads, which can record images over the width of an A-4 size sheet (Japanese Standard, 210 mm) at one time.
FIGS. 8A to 8D are schematic views of such a line head 1. In FIGS. 8A to 8D, electric connections to head chips 4A to 4D are eliminated. Proportions in thickness and length of components are different from facts in the drawing for description convenience sake. Also, the line head for the A-size has the 64 head chips as mentioned above; however, for simplification, the four head chips 4A to 4D will be described with reference to FIGS. 8A to 8D. Referring to FIGS. 8A to 8D, the line head 1 includes a nozzle plate 3, four head chips 4A to 4D and six dummy chips 5A to 5F, which are bonded on one surface of the nozzle plate 3, and a flow path plate 2 formed further over these chips.
FIG. 9 is a sectional view showing the flow path plate 2, the head chip 4, and the nozzle plate 3 in detail. As shown in FIG. 9, the head chip 4 has heater elements 4b arranged on a semiconductor substrate 4a. At 600 DPI, the 320 heater elements 4b are arranged for one head chip 4. On the surface having the heater elements 4b arranged thereon, a barrier layer 4c is laid so as to form the liquid chamber.
The nozzle plate 3 has an arrangement of nozzle openings 3a formed therein at positions corresponding to those of the heater elements 4b of the head chip 4.
In the example shown in FIGS. 8A to 8D, the head chips 4 are arranged in a staggered form. Between the head chips 4, the dummy chips 5 are arranged substantially without clearance (between the head chips 4A and 4C, the dummy chip 5C is arranged, for example). The dummy chip 5 is the same as the head chip 4 at least in height, and it may have the same shape as that of the head chip 4 and may not have the heater elements 4b. The dummy chip 5 does not eject ink.
Furthermore, the dummy chips 5A and 5F among the dummy chips 5A to 5F are arranged at both ends of the head chips 4A to 4D in the longitudinal direction, so that a liquid supply path 2a is surrounded with the head chips 4A to 4D and the dummy chips 5A to 5F. Also, the head chips 4A to 4D and the dummy chips 5A to 5F form a flat surface on which the flow path plate 2 is bonded.
The flow path plate 2 includes a liquid inlet 2b formed at the upper center and the liquid supply path 2a formed inside the flow path plate 2 so as to communicate the liquid inlet 2b and the head chips 4.
Referring to FIG. 9, when the heater element 4b arranged on the head chip 4 is heated, bubbles are produced on the heater element 4b. Although the bubbles diminish within a short period of time, a soaring force is applied to liquid on the heater element 4b by pressure changes due to generation/extinction of the bubbles at this time. Then, by the soaring force, liquid droplets are ejected from the nozzle opening 3a. 
The heat in the head chip 4 is almost generated from the heater element 4b. Furthermore, even on the side of the heater element 4b, with which liquid is not brought into contact, the heat produced from the heater element 4b is transferred because the heater element 4b comes contact with the semiconductor substrate 4a. 
The heat produced in the head chip 4 is transferred to the liquid moving every ejection of liquid droplets. In other places, the bottom surface of the head chip 4, for example, the heat is transferred to the flow path plate 2 via an adhesion layer 6 between the head chip 4 and the flow path plate 2, and in the front surface of the head chip 4, the heat is transferred to the nozzle plate 3 via the barrier layer 4c of the head chip 4.
However, the conventional technique described above has the following problems in a practical application.
As the single head chip 4 is about 20 mm in size as mentioned above, even when the head chip 4 has the nozzle plate 3 with the nozzle opening 3a and the flow path plate 2 bonded thereon, if strain is generated by the thermal stress between components due to thermal expansion, the stain is not at the level to a failure in a serial system.
On the other hand, when a number of the head chips 4 are connected together like in the line head 1, as the length in the longitudinal direction is increased, the expansion difference due to thermal expansion, i.e., the difference between linear expansion coefficients becomes a problem depending on materials arranged on the front surface of the head chip 4 (the side of the nozzle plate 3) and on the bottom surface (the side of the flow path plate 2).
If materials of the flow path plate 2, the head chip 4, and the nozzle plate 3 have substantially the same linear expansion coefficient, the thermal expansion problem does not arise. However, upon selecting materials of the flow path plate 2, the head chip 4, and the nozzle plate 3, characteristics or functions required for each member are different, so that each member must satisfy the required characteristics or functions.
For example, for the flow path plate 2, cast aluminum is given at first. This is because of its excellent workability and thermal conductivity. Then, an injection-molded acrylic resin is given. This is because of its excellent wettability and workability as well as lower Young's modulus in comparison with aluminum.
Furthermore, for the barrier layer 4c, a high-polymeric material, typified by a photosensitive cyclized-rubber resist or an exposure-curing dry-film resist, is shown. This is because of its strong adhesive force, higher hardness after cured than that an acrylic resin, and low cost.
Also, as the nozzle plate 3, electrocasting nickel is given because the nozzle opening 3a is comparatively simply constructed by that, its thermal expansion is comparatively small, as well as its wettability and cost are within a practical application.
As described above, each member must select a material as well as a fabricating method so as to satisfy characteristics or functions required for each member. When materials of the flow path plate 2, the head chip 4, and the nozzle plate 3 are selected in such view, linear expansion coefficients thereof are to be different from each other.
FIGS. 10A to 10C are sectional views illustrating generation of thermal stress and strain in the line head 1, wherein FIG. 10A qualitatively shows the extent of displacement due to temperature changes. In the drawing, the center of the line head 1 in the longitudinal direction is established to be an original point. In this case, with increasing temperature, the nozzle plate 3 and the flow path plate 2 are elongated so that the closer to both ends from the center, the displacement becomes larger relative to the position before temperature rise, as shown in the drawing. The length of arrow indicates the magnitude of its displacement.
FIG. 10B is a sectional view showing an example of deformation due to temperature change. When linear expansion coefficients of the flow path plate 2 and the nozzle plate 3 are different from that of the head chip 4 (those are larger than that of the head chip 4, in this example), the flow path plate 2 and the nozzle plate 3 are to be elongated longer than the length of the line of the head chips 4, and are warped like an arrow in the drawing as a bimetal phenomenon if between the flow path plate 2, the head chip 4, and the nozzle plate 3 are bonded together with an adhesive while other parts are free.
When the line head 1 is warped like an arrow in such a manner, the distance between a recording medium and each head chip is changed. For example, in the head chips 4 located at both ends, the distance between the nozzle plate 3 and the recording medium is not so changed; however, the head chip 4 is inclined (not in parallel) to the recording medium. On the other hand, in the head chips 4 located in the central portion, with the line head 1 warped like an arrow, although the parallel is not so changed, the position of the head chip 4 is moved upward, so that the distance to the recording medium is elongated.
Then, in order to prevent the deformation like an arrow, the positional relationship between the line head 1 and a recording medium is maintained by applying a force to the line head 1.
As shown in FIG. 10C, the line head 1 is pressurized at the central portion from the top while being supported at both ends from the bottom by applying forces F1 to F3 thereto, so that the deformation like an arrow can be suppressed (evenness is maintained).
In this case, however, shear stresses are produced between the flow path plate 2 and the head chips 4 and between the head chips 4 and the nozzle plate 3, as shown by arrows in the drawing, and the closer to both the ends, magnitudes of the shear stresses are increased.
In particular, on the head chip 4, the barrier layer 4c is laid as mentioned above so as to form a liquid chamber and an individual flow path with the barrier layer 4c. The strength of these portions is smaller than that of the semiconductor substrate 4a of the head chip 4 or the nozzle plate 3 so as to cause elastic deformation and plastic deformation due to the shear stress, so that it may be difficult for the liquid chamber and the individual flow path to satisfy the required characteristics.
FIGS. 11A and 11B show pictured results of a liquid ejection part of the line head 1 when such thermal stress is applied thereto, wherein FIG. 11A shows the central portion of the line head 1.
As shown in FIG. 11A, deformation (strain) scarcely exists. Whereas, as shown in FIG. 11B, at both ends of the line head 1, the barrier layer 4c is deformed so as to possibly affect ejection characteristics.
For reducing such effect, in a general operating proof temperature range of a printer, such as a range between 15 to 35° C., changes in ejection characteristics need to be further reduced to temperature changes.